Magnetic-disk drives generally utilize rotary actuators to position a stack of magnetic read-and-write heads (also known as transducers) with respect to an array of magnetic disks rotationally mounted on a spindle. A read-and-write head is moved to a particular track of a magnetic disk to gain access to the information recorded on that track.
A rotary actuator of a magnetic-disk drive usually comprises a housing, a shaft that is pivotally installed within the housing on precision bearings, and an array of stacked arms for supporting the read-and-write heads, i.e., a head-arm assembly. The head-arm assembly is keyed to, or is integral with, the shaft. Typically, a voice-coil motor pivots the head-arm assembly to position the transducers at selected radii (or tracks) of the magnetic disks.
However, the pivoting mechanism of the above-described actuator has a number of inherent flaws, one of which is the internal resistance in the bearings that support the shaft. Since the inner races of these bearings are usually bonded to or have an interference fit with the actuator shaft and the outer races are bonded to or have an interference fit with the actuator housing, it is difficult to control the bearing load, i.e., the residual force directed along the axis of the shaft. Temperature-induced dimensional variations within the actuator assembly multiply existing bearing loads, creating rotational resistance.
Accordingly, as bearing loads go up, the amount of torque required to pivot the actuator shaft, and hence the head-arm assembly, becomes greater. Also, additional energy is needed to overcome rotational drag produced by the bearing lubricant. The increase in resistance encountered by the voice-coil motor causes the positioning error of the transducers to rise proportionally. Excessive positioning error induces data corruption, i.e., recorded data cannot be recovered or new data is written undesirably and destructively. Furthermore, mechanical resistance in the pivoting mechanism of the actuator limits the storage capacity of the disk drive and increases power consumption.
Another shortcoming associated with the pivoting mechanism is the low-frequency resonance of the bearing balls as the actuator imparts small angular displacements to the head-arm assembly. During such displacements, the bearing balls deflect instead of rotating, thus producing low-frequency resonant vibration that further impairs positioning accuracy, data-access speed, and memory-storage capabilities of the disk drive.
As an added disadvantage, actuator bearings must be precision made to satisfy the critical positioning requirements of the read-and-write heads. The cost of these exacting components greatly increases the expense of disk-drive assemblies. Moreover, actuator bearings cannot withstand external shocks a disk drive may experience when transported.